Log Message:
make unregistered known controls show up as undefined; move LDAPsync
cid to frontend; some cleanup

Unfortunately this patch has broken test015 (control not available in
context) because the backends reference slap_known_controls, not
slap_known_controls+1.

I think we can still use this idea, just using another offset. If we
define slap_known_controls to point to the second element of an array,
we can use "-1" as the undefined initializer and array[-1] will be the
unused/NULL entry.